1. Field of Invention
The present invention relates to cellular communications; and more particularly to a method and apparatus for handing off on-going communication between cells.
2. Description of Related Art
A cellular communication system includes several base transceiver stations, each of which transmits and receives channels of RF information throughout a predetermined coverage area referred to as a cell. The outer boundary of a cell is determined not only by the effective radiated power of the base transceiver, which determines the maximum range at which a mobile unit is able to receive; but also, by the effective radiated power of the mobile telephones, which determines the maximum range at which the base station is able to receive. The base transceiver stations (BTS) determine their corresponding cell's handoff boundaries by setting an RF signal threshold value which must be met or exceeded by a mobile unit entering the cell in order to transfer an on-going call to that cell.
Based upon the received RF signal strength of the mobile unit by the transceiver of the serving cell, the BTS controls the effective radiated output power (ERP) of the mobile unit so that the transmission by the mobile unit does not saturate the base station transceiver when it is close to the center of the cell, but is of sufficient power to enable the BTS to receive the mobile transmission when it is far from the transceiver. When the RF received signal strength of a traffic channel of a neighboring cell measured by the scanning receiver in the serving cell exceeds the set RF signal threshold value that is configured into the system; then the system switches on-going communication to a traffic channel of the neighboring cell, thus, effecting a handoff.
The base station equipment for each cell is structured to handle an expected maximum number of concomitant cells involving mobile units located within a specific geographic area. Should this maximum number be reached, no channels are available; and a newly initiated call is blocked, or an on-going call is lost during an attempted handoff to the congested cell.
Many geographic areas, particularly large metropolitan locations have many mobile phone units in operation at any one time. This, of course, results in the aforementioned call blocking as well as interference when the loading in a particular cell becomes excessive. To avoid the excessive loading of an individual cell, it has been proposed to divide the metropolitan area into several small neighboring cells. This, of course, increases the complexity and amount of equipment that must be installed at the base station of each of the small neighboring cells, particularly scanning receivers for measuring the signal strength in order to effect a handoff between the neighboring cells.
Thus, there is a need for a method and system to accommodate the large number of mobile units in use at a given time without the necessity of increasing the complexity and amount of equipment to effect a handoff.